C3-V3.4: Mechanical anchorage
Table of contents
Project data
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Report in the annual report 2020
MECHANICAL ANCHORAGES FOR CRC
Storage of the carbon concrete specimens at the outdoor weathering stand of the Institute for Corrosion Protection Dresden
A main goal of the project is the development of mechanically acting anchorage structures for carbon fibre reinforced concrete (CRC) with high effectiveness to improve the bond between the carbon textile and the concrete and to reduce the anchorage length. One approach were polymer coatings and small anchoring structures applied on the yarn. Furthermore, curved and shaped grid structures were investigated, which were used to mechanically anchor textile carbon reinforcement. In tests with modified ends of grids in the end anchorage area, a load increase was measured after activation of the folded area of the grid. For folding, a polymer impregnation was specifically developed with regard to the bond requirements and the thermal formability of the textile. The tests showed that the greatest potential for optimizing the bond lies in exploiting the mechanical form-fit of the yarn shape and ensuring this through sufficiently stiff polymer impregnation.
The 2nd research aspect of the project is corrosion in CRC construction. Therefor, specimens were made of the hybrid material combination concrete–metal–carbon and exposed to different environmental stresses. The focus was explicitly on contact corrosion between metallic elements and carbon reinforcement. With various methods, such as rapid carbonation, the specimens were accelerated aged. These specimens were subjected to alternating stresses in a climatic chamber as well as to real conditions of outdoor weathering. After the corrosion tests, the specimens were broken up and the corrosion progress on the metallic elements was documented. Under certain conditions,
corrosion could be observed on the metallic elements, particularly on specimens with unalloyed elements and a high chloride exposure as well as carbonated concrete. Cracks and pores in the concrete as well as the direct contact of metal with the much more noble carbon reinforcement had an accelerating effect. Corrosion of stainless steel elements was also observed in some cases. Based on the results, it can be concluded for the tested
material combinations that contact corrosion of the metallic elements can and should be prevented by design measures. Dense, high-strength concretes can significantly slow down corrosion processes by making it more difficult for moisture and other substances to penetrate. However, the prevention of direct contact makes sense in any case and is necessary to maintain durability.
Report in the annual report 2019
MECHANICAL ANCHORING
The C3-V3.4 project investigates mechanical anchoring structures that are supposed to improve the bond between carbon fibres and concrete and, as a result, reduce anchorage length. The ambition is to modify certain areas of the textile reinforcement to create local short-length anchorages while maximizing utilization of material. Additional topologies, which are applied to the textile, also secure anchorage by utilizing their geometry.
Mechanical anchoring structure in bond between carbon fiber yarn and concrete
The preliminary considerations yield many parameters that influence the form and dimension of those structures. The dimension of those structures, in turn, is limited by the geometry of the textile grid and the desire for a thin concrete covering. The durability of the bond between structure, concrete and carbon fibre also has to be ensured, which results in certain requirements concerning material and application of geometrical structures.
Carbon grid with additional polymer coating to improve the bond behaviour
The experiments were based on a flexible textile, which was developed further in cooperation with the project partners. Main aspects were flexibility and the requirements for composite properties. Several approaches for different anchoring structures (form, material, type of application) turned out to be promising and resulted in an increase of composite characteristics. In collaboration with the project partners, they are developed further, and solutions for mechanically anchoring textile reinforcement within the concrete are found.
For example, there is a carbon grid under development which is capable of being formable during heat treatment. Hence single areas of the textile reinforcement can be adjusted to match certain geometries (e.g. folds, loops), thus providing mechanical anchoring within the concrete due to their shape.
Report in the annual report 2018
INCREASED COMPOSITE RESISTANCE BY SHAPING
Auf den Garnenden applizierte Verankerungskörper
Reinforcing steel has become indispensable in the construction of concrete structures. The technology has been used and researched since the middle of the 19th century. At present, the bond between the two material components concrete and reinforcement steel is no longer only ensured by hooks and loops in the component ends, but also by rolled up or milled ribs. The shape and arrangement of these geometrical changes, which improve the composite, have been optimised over decades up to the present design.
The bond between the carbon reinforcement and the concrete in the still young textile or carbon reinforced concrete material combination has mainly been realised with the help of a polymer matrix which connects the carbon fibres to fibre strands, and has been continuously improved by modifying these. The C3-project “Mechanical anchoring” investigates an approach to improve the bond between concrete and carbon reinforcement by modifying the shape of the textiles, similar to reinforcing steel ribs in reinforced concrete construction. For this purpose, comprehensive experimental studies will be carried out on the bond behaviour and various methods to improve the bond will be tested, which are based on the principle of mechanical bond. These include, for example, glued polymer anchoring bodies or structured fibre strand surfaces.
In addition to the experiments carried out in the Otto Mohr Laboratory, the various possibilities will be investigated in simulation models. These observations are based on reference experiments which can be used to identify and evaluate the improvement and/or deterioration of the bond behaviour in the individual phases.
Plastische Dehnungen im Betonmodells während des Garnauszugs
The simulation is realised as a three-dimensional structural model with volume elements representing the two materials. The bond is generated by contact surface elements. Depending on the transverse compressive force acting on the respective surface and the defined coefficient of friction, a maximum adhesive tensile stress is calculated. If this stress is exceeded, the surfaces glide past each other. This makes it possible to create the bond stress-slip curves that are important for evaluating the bond behaviour and are comparable with the experiments carried out in the laboratory. Since the computer simulation allows a look into the material, it provides a high gain of knowledge about the bond mechanisms acting in the composite material.